In a PCB on which a LSI is mounted, there is a structural problem that a power supply voltage of the PCB fluctuates because an electric current produced by operation of the LSI flows into a power supply of the PCB, and the LSI does not operate normally by the voltage fluctuation. Therefore, in order to operate the LSI normally, trials to suppress the fluctuation of the power supply circuit voltage are performed such as reducing an electric current flowing into a power supply circuit (hereinafter, described as “power current”) of the PCB including the LSI or reducing the impedance of the power supply circuit.
For example, the structure is described in the patent literature 1 which includes a circuit for detecting a voltage drop (fluctuation) and a regulating circuit for adding voltage in an LSI (block). The voltage drop detection circuit calculates the voltage drop amount arising in the LSI operating time and feeds its voltage drop amount back to a voltage regulating circuit. A semiconductor integrated circuit design device is described which supplies the stable voltage of power supply by including two above-mentioned circuits in an LSI and setting the voltage arising due to a voltage drop to the operating voltage of the regulating circuit.
The semiconductor integrated circuit design device is described in the patent literature 2 which includes a means for analyzing noise, a means for judging whether the noise level is within the predetermined range, and a means for adding a bypass condenser to the inside of the circuit block of LSI with the noise out of the predetermined range.
Its feature is that it includes a noise analyzing means for performing a noise analysis from the information in a database, a logical gate selection means for searching a logical gate with the noise generation exceeding the predetermined range if the amount of the noise exceeds the predetermined range, and a bypass condenser addition means for adding a bypass condenser for reducing power supply noise and substrate noise to the selected logical gate.
By those means, it is possible by using the information from a database to add a bypass condenser for reducing noise to the neighborhood of the logical gate circuit with a large amount of noise which can become a noise source, and suppress the noise efficiently.
Since it is possible by having set the predetermined range of the noise to find automatically a logical gate of which a bypass condenser should be added to the neighborhood, the LSI structure is provided which is designed to be stable for noise by a series of automatic flows.